At two sites in Singapore and Malaysia that had bleached in 1998, this pattern was reversed, with normally susceptible Acropora colonies appearing healthy while massive slow-growing corals, such as Porites were heavily damaged.
Pictures of ghostly white coral colonies bleached by elevated sea temperatures have become symbols of the effects of global warming. Now there is a glimmer of hope that at least some corals may be more resilient than previously thought. A study suggests that certain kinds of corals subjected to bleaching adapt to endure higher water temperatures.
Survivor. Healthy Acropora (in front) appear to have developed resistance to high sea temperatures while bleached Porites did not. Photo Credit: James Guest
Corals rely on symbiotic algae, called zooxanthellae, for their color and to produce nutrients through photosynthesis. Above a tipping point, warm seawater typically upsets this delicate symbiotic balance and corals expel the algae, turning white and eventually dying if high temperatures persist. Such bleaching events are becoming more frequent as periodic hot spells exacerbate the sea temperature rise due to global warming. This raises concerns about the long-term survival of coral reefs, which are refuges for marine biodiversity.
Yet corals may be hardier than biologists have thought. During a 2010 bleaching episode, an international team studied three coral reef sites. At one in Indonesia that had not bleached previously, corals responded typically to warmer water. There, fast-growing branching coral species—such as Acropora—suffered severe die-offs. But at two sites in Singapore and Malaysia that had bleached in 1998, this pattern was reversed, with normally susceptible Acropora colonies appearing healthy while massive slow-growing corals, such as Porites were heavily damaged. The group concluded that "the effects of bleaching will not be as uniform as anticipated" and fast-growing corals such as Acropora and Pocillopora may be able to survive more frequent rises in water temperature. Marine biologist James Guest, previously at the National University of Singapore and now at University of New South Wales in Sydney, Australia, and colleagues reported their findings online on 9 March at PLoS One.
The report "is very interesting and hopeful," says Mikhail Matz, a coral biologist at University of Texas, Austin. Matz says it appears natural selection led to the evolution of higher bleaching resistance in just one coral generation, "which would be awesome news indeed." He would like to see additional evidence to clarify the mechanism involved.
Guest agrees additional work is needed. "We don't know whether the unusual resistance in the branching corals was due to the host coral or the symbionts or both," he says. They are starting additional studies to learn more about the specific type of zooxanthellae inhabiting the coral that adapted and to try to study the phenomenon in the laboratory. He also cautions that higher water temperatures could still affect the composition and health of reefs. Finding evidence of adaptation "does not mean that the global threat to reefs from climate change has lessened," he says.
Global Warming Threat to Coral Reefs: Can Some Species Adapt?
ScienceDaily 10 Mar 12;
Coral reefs are among the ecosystems most severely threatened by global warming, but hopeful new evidence has emerged that some coral species may be able to adapt to warmer oceans.
In a study published in the journal PLoS One, an international team of researchers reports that coral populations which unexpectedly survived a massive bleaching event in 2010 in South-East Asian waters had previously experienced severe bleaching during an event in 1998.
The team analysed what happened at three sites during the 2010 event and found that in Indonesia, corals responded to higher sea temperatures in a typical way, with fast-growing branching species -- such as staghorn corals -- suffering severe die-offs. But at sites monitored in Singapore and Malaysia, the usual trend was reversed: normally susceptible colonies of fast-growing Acropora corals appeared healthy and fully pigmented, while most colonies of massive coral were severely bleached.
"Mass coral-bleaching events, caused by a breakdown in the relationship between the coral animals and their symbiotic algae, are strongly correlated with unusually high sea temperatures and have led to widespread reef degradation in recent decades," notes lead author Dr James Guest, currently a joint research fellow at the UNSW Centre for Marine Bio-innovation and the Advanced Environmental Biotechnology Centre at Singapore's Nanyang Technological University.
"The severity of these events varies considerably but until now we've seen one consistent trend: certain types of coral tend to be more resistant to bleaching than others. This has led to the prediction that hardier, slow-growing massive species will replace less hardy, fast-growing branching species on reefs in the future.
"But during the 2010 event the normal hierarchy of species susceptibility was reversed in some places. Corals at our Indonesian study site in Pulau Weh, Sumatra, followed the usual pattern, with around 90% of colonies of fast-growing species dying. But the pattern was the opposite at study sites in Singapore and Malaysia, even though sea-temperature data showed that the magnitude of thermal stress was similar at all sites.
"This suggests that the thermal history of these sites may have played an important role in determining the bleaching severity in 2010."
Journal Reference:
James R. Guest, Andrew H. Baird, Jeffrey A. Maynard, Efin Muttaqin, Alasdair J. Edwards, Stuart J. Campbell, Katie Yewdall, Yang Amri Affendi, Loke Ming Chou. Contrasting Patterns of Coral Bleaching Susceptibility in 2010 Suggest an Adaptive Response to Thermal Stress. PLoS ONE, 2012; 7 (3): e33353 DOI: 10.1371/journal.pone.0033353